ELECTRETS FOR MEASUREMENT OF BACK-GROUND RADIATION DOSE AND RADON CONCENTRATION IN AIR

by

S.O. Soman Health Physics Division, Bhabha Atomic Research Centre Bombay 400 085 lndia

ath Communication within the Bilateral lndo-German Scientific Agreement on "Advanced Aspects of Trace and Ultratrace Analysis: Trace Analysis of Radionuclides in the Environment"

Table of Contents

Abstract page

5

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1 . lntroduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2. Preparation of Electrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3. Handling of Electrets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0

4. Charge Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 0

5. Background Radiation Dose Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

6. Measurement of Radon Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

7. References ................................................................ 19

Abstract

A survey is given about investigations on electrets and their application in environmental radioactivity measurement, which has been performed for the last few years in the Health Physics Division of the Bhabha Atomic Research Centre, Bombay, lndia.

This survey includes preparation, handling, charge reading and application of electrets for background radiation dose as weil as radon concentration measurement in air.

Zusammenfassung

Es wird eine Übersicht gegeben über Arbeiten der letzten Jahre in der Health Physics Division des Bhabha Atomic ResearchCentrein Bombay, Indien, über Elektrete und deren Anwendung zur Messung der Umweltradioaktivität

Der überblick enthält die Herstellung, Behandlung, Ladungsablesung und Anwendung der Elektrete für Messungen der Untergrund-Strahlendosis sowie Radonkonzentration in der Luft.

5

Preface

Electrets are a counterpart to permanent magnets in the field of electrostatics. They can be prepared by synthesizing specialplasticssuch as teflon and its similar compounds under the influence of an electrostatic field. The molecular electrical dipals became straightend into one direction, which is fixed after solidifying of the resin. This electrets can be discharged by the up-take of electrical charges with opposite sign from the environment, which may e.g. be induced by ionizing radiation. On the reverse, this radiation can be measured by following the discharge of an electret.

This article was a lecture at the Seminar for Ghemical Analysis of the KFA on the 25th November 1982 under the Bilaterallndo-German Scientific Agreement on "Advanced Aspects in Trace and Ultratrace Analysis: Trace Analysis of Radionuclides in the Environment" between the Health Physics Division of the Bhabha Atomic Research Gentre (BARG) in Bombay, lndia, and the Gentral Department for Ghemical Analysis of the Nuclear Research Establishment Jülich (KFA). This Project has been described in the preface to the previous JOL-Spez-Report No. 161.

Mr. S.D. Soman is Head of the Health Physics Division of the BARG. He gives in the following a briet summary on investigations from his Division about the application of electrets for doserate measurements of ionizing radiation.

Electrets are a new and promising tool for measuring environmental radioactivity as radiation doses and dose rates under extremely simple experimental conditions.

B. Sansani

Jülich, 24th August 1982

6

1 . lntroduction

An electret is a piece of dielectric material exhibiting quasi-permanent electric charge. The charge of the electret produces a strong electrostatic field capable of collecting ions of opposite sign. Marvin 111 was the first to suggest that the reduction of charge on electret was due to collec-tion of ions of opposite sign from the surrounding gas and proposed electret as a dosimeter. Wolfsan and Dyment l21 noted that the reduction of charge was not stable for Carnauba wax electrets after the termination of irradiation. Until recently electrets were regarded merely as curious analogues of permanent magnets, worthly only of academic interest. However, with the advent of the development of fluoro-carbon polymerssuch as Teflon, electrets have finally be-come accepted as reliable electrostatic components capable of maintaining a constant elec-trostatic field even at high temperatures and severe humid conditions. Electrets made from Tef-lonshow remarkable stability, with mean life of tens of years, and have ability to retain charge in the most adverse conditions likely tobe encountered in the environment. Technology for mak-ing electrets is also fairly weil understood and so also the techniques for measuring the surface charge of the electrets. Bauser and Ronge l31 made significant advancement in the use of elec-trets for radiation dosimetry. They used a pairofthin Teflon electrets of opposite charges to make an ionisation chamber. Such an ionisation chamber allows quick and repetitive read out linear to dose and measures the cumulative dose without loss of information. They claim a threshold dose of about 0.01 mGy. Electrets have also been used for measuring concentrations of radon and thoron l41 gases in air.

ln the Health Physics Division at BARG, we have carried out the following programmes:

• Development and standardisation of techniques for making electrets;

• Design and fabrication of a portable unit for reading surface charge of the electrets;

• Design and development of a single electret dosernster and study of its performance for ex-ternal dose; and

• Development of a method for measurement of radon concentration using electrets.

7

2. Preparation of Electrets

Two methods have been standardised:

2.1 Preparation of Thermo-electrets l41

A circular piece (6 cm in diameter and 0,08 cm thick) of Teflon coated with a thin layer of aluminium on either side was made into an electret. This sheet was sandwiched between two polished stainless steel discs, and lowered into an oven maintained at 240°C. A high voltage of about 4.5 kV was applied between the discs and the sandwich was allowed to cool to room temperature with the electric field still on. The high voltage was switched off, the sandwichwas removed and held shorted (using metal clip) for about 10 days to remove unstable charges. The aluminium coating was removed and the Teflon sheet was washed, dried and retained shorted between the metal sheets for use as an electret. The Teflon electret thus prepared had acharge density of 7 x 1 o-9 coulomb/cm2 as measured by a standard shutter method. The chargewas equal and opposite on either side. This electret produced an electric field equivalent tothat pro-duced by a battery of nearly 3000 volts.

2.2 Liquid Contact Method l51

ln this method a filter paper F wetted with a few drops of electrolyte such as dilute HCI was sandwiched between the metal piston and Teflon sheet with its other side having conducting surface. The voltage of 3 to 5 kV was applied between the metal piston MP and the conducting side CS (Figure 1). After about five minutes the metal pistonwas removed out of the Teflon sheet with the high voltage still on to make the Teflon into an electret. Then the high voltage was switched off. To prepare good quality electrets it is necessary to pre-heat the Teflon sheet and also to give heat treatment after preparation.

8

HV

MP

GP Figure 1: Liquid Contact Method for Making Electrets (Exploded View) Dimensionsare in mm; MP metal piston; I insulating layer; HV high voltage unit; F electrolyte socked filter paper; T teflon sheet tobe made into an electret; CS graphite coated conducting surface; GP grounding plate.

9

3. Handling of Electrets

ln view of the inherent properties of electrets, certain handling problems arise. Due to large electrostatic fields, electrets attract dust as weil as ions of opposite sign present in the environ-ment. lf an electret is left in dusty atmosphere, a thin layer of dust collects on its surface. This reduces the electric charge. The thermo-electret can be cleaned with soap and water to regain its original electric charge. The cleaning, however, should not be done with abrasive powders. The electrete prepared by liquid contact method should not be washed since these lose their charge. Electrets when not in use should be kept covered insmall plastic containers to minimise collection of ions and dust.

4. Charge Reading

Capacitive probe method described by Sessler and West rsJ (Figure 2), also known as the sutter method, was adopted for measuring the charge on electrets.

Electret E loaded in an electret holder (Figure 3) was positioned and the shutter pas pushed into the slot. The probe was momentarily shorted and the display zeroed. The shutter was pulled out and the maximum voltage recorded on the displaywas noted. Using various parameters of the system, the surface charge of the electret could be calculated l5l. The measuring system is small, battery operated and portable.

10

D B A

p ss

-ds es Figure 2: Schematic of Charge Measuring Unit

P Probe SH Shutter that can be pushed or withdrawn through a slot GR Guardring E Electret CS Conducting surface of electret dS Distance between the probe and the electret de Thickness of electret C Capacitance SS Shorting switch A High input impedance stage (MOSFET) 8 Differential amplifier using operational amplifier D LCD digital display

11

/ k--------- ----68 --~-~ I

6

--74

J ___ _

~- -------- - ------------76 ------~ 2

l 30

oc~-

G L ~---------j ____ --'.~~~~~

3 _ _r-- - __ _1

Figure 3: Exploded View of Dosemeterand Electret Holder.

Dimensions are in rnm H Handle for Iid L Perpex Iid to screw into EH es Graphite coated conducting surface of electret E DC Dosemeter chamber GL Thin graphite layer TM Threaded to mach

12

5. Background Radiation Dose Measurements

The dosemeter standardised in the Iabaratory is shown in Figure 3. Electret in an electret holder was read before loading into the perspec chamber. The assembled perspec chamber is called dosemeter, and the reading of the electret involved is called the dosemeter reading.

The system is very nearly air equivalent. The dosemeterwas given doses in steps of 1 mGy, and the reading after each successive dosewas taken. The response of the dosemeter in terms of volts per mGy over a range of dosemeter reading was like a typical ionisation chamber re-sponse with a saturation region between 5 and 20 volts (Figure 4). The saturation region cor-responds to a response of about 0.8 volts per mGy for an electret of 0.08 cm thickness.

lt is seen that the experimentally measured response per milli Gray agrees fairly weil with the calculated response based on instrument parameters. lt is also independent of doserate and photon energy. The response of dosemeter of electrets of different thicknesses was also studied. Useful range, minimum resolvable dose and the response for different electret thick-nesses are given in Table 1.

Table 1 : Characteristics of Dosemeter

Electret Response Minimum Dose Useful range of Maximum Thickness (volt/MGy) Resolvable Dosemeter Measurable

(cm) (mGy) Readings (volts) Dose (mGy)

0.0127 0.1341 0.0751 5.2 to 20.8 116 0.0794 0.7981 0.013 5 to 20 18.8

0.3084 2.6615 0.0038 4.5 to 17.8 5.0

lt is seen that for an electret of 0.3 cm thickness, the minimum resolvable dose is 0.0038 mGy. This corresponds to a value of 0.01 volt background radiation measurements.

13

1.10 ..------------------------=.=-----,

t ->-l!)

E

0::: w CL

(./) t-_.J

0 > -LlJ ~ z 0 CL (./) w 0:::

0.10

0

•

CL....-----'---------I.----l...-----L---L---__L_----1

0 5 10 15 20 25 30 35

DOSEMETER READINGS IN VOLTS~

Figure 4: Experimentally Determined Response (volts/mGy) of Measuring System for Various Dosemeter Readings (volts).

Electret thickness: 0.0794 cm Dosemeter exposed to 192 Ir Photons Dose rate: 45 mGy/hr

14

6. Measurement of Radon Concentration

The decay products of radon carry positive charge immediately after their formation. These daugther products can, therefore, be quantitatively collected on metal electrodes maintained at high negative potential. The Teflon electrets by virtue of their high charge density can be used to collect these daugther products. Figure 5 shows the 1 0 I chamber used for measurement of radon concentration.

The decay products formed inside the chamber were collected on to the surface of the mylarfoil covering the elctret. After a known sampling time, the mylar sheet was alpha counted. The theory analogaus to the theory of doublefilterwas used to calculate the concentration of radon in air. ln case of radon in about 8 minutes, 90 % of equilibrium is attained between the volume inside the chamber and the environmentoutside whereas in about 20 minutes full equilibrium is established. Thus for a few hours of sampling time this effect is negligible. However, due to short half time of thoron a gradient of concentration is established between the wall and the centre of the chamber and concentration in the chamber is lower than the outside concentration by a factor which depends on the area of the chamber available for thoron to diffuse in.

The relative humidity (RH) has a considerable effect on the collection efficiency of radon decay products [41. As RH increases from 1 0 % to 75 %, the collection efficiency decreases from 70 to 40 %. This effect is less pronounced for thoron decay products. These Observationsare similar to those of Cowper and Davenport [7 for radon daughters and of Porstendörfer and Mercer [SJ for

thoron daughters.

lt is possible to calculate both radon and thoron concentrations by programmed alpha counting of collected decay products. For three hour sampling and subsequent counting the minimum detectable Iimit works out to about 30 pCi/m3 for radon and 300 pCi/m3 for thoron.

Table 2 shows that the method is capable of measuring the Ieveis encountered in the environ-ment and the results agree fairly weil with measurements carried out using large 150 litre dou-

ble filter system.

Long term cumulative concentrations of radon or thoron can also be measured by collecting decay products directly on to the surface of TLD or SSNTD[91 • The arrangement for using an electret for such purposes needs some modifications. These configurations are shown in Fig. 6. lt is seen that the TLDs registered nearly 0.0137 mGy per pCill.hr for radon and 0.0007 mGy per pCi/l.hr for thoron. ln the case of SSNTD using CR-39 the detector recorded 90 ± 1 0 tracks/cm2 per pCill.hr for radon and 6 ± 0.06 track/cm2 per pCi/l.hr for thoron. Simultaneaus measurements of radon and thoron by this technique is possible if one uses two chambers with different thicknesses of foam and hence different responses.

15

r--HOOK FOR HANGING

~------ 294 -------~

S' ~ 60 ~ 71 . L r- ~

I p - ' ..

TEFLONj ~ELECTRET ..

.. . . .

GASKET .. .. ELECTRET . ~ ... HOLDER

.. 14 7 ...

. . .

s.s. 70 -. .. MESH WIRE SCREEN - .. . . . ..

.. ,,, . . . . . .. : .. . ......... .. . . . . . .. •: I

' · ... . . . . . . . . .. . . . . . . . .. . . . . : . . • . . ... . . . . . . .. .. . . . .. . . . . . . . .___ POLYURETHENE FOAM

Dimensions are in mm.

Figure 5: Electret Chamber System for Measuring Concentration of Radon and Thoron

16

Table 2: Measurements of Concentration of Radon and Thoron in Unventilated Laborstory Room by Electret Chamber System and Double Filter System

Radon (pCilm3)

Electret Double Date System Filter

Ce System

Cd

Feb 11 486 333 Feb 12 454 480 Feb 13 469 539 Feb 14 371 268 Mar 11 366 245 Mar 12 314 327 Mar 13 460 391 Mar 14 507 410 Mar 15 464 447 Mar 18 274 240 Mar 19 182 182 Mar 20 221 257 Mar 21 438 258

Mean and standard deviation of Cd/Ce = 0.89 ± 0.19

Cd

Ce

0.69 1.06 1.15 0.72 0.67 1.04 0.85 0.81 0.97 0.88 1.00 1.16 0.59

Thoron (pCi/m3 )

Electret Double Cd System Filter

Ce System Ce Cd

1371 1261 0.92 586 498 0.85 640 618 0.97 - - -

1954 2605 1.33 2651 3000 1.13 1042 1968 1.89 1542 909 0.59 769 979 1.27

1029 767 0.75 1449 2412 1146 1537 1605 1607

Mean and standard deviation of Cd/Ce = 1.14 ± 0.38

1.66 1.34 1.00

Note: Errors due to counting statistics werein the range of 5 to 10 %in all the measurements. Relative humidities were in the range of 50 to 60 % and a corresponding value for Fe was used in the calculations.

17

®

Figure 6:

CHARGE GONFIGURATION

INDUCED + Ve CHARGE Al DISC FOR TLD

ELECTRIC

®

ELECTRET HOLDER

©

SSNTD (CR-39) ALUMINIZED

60 mm DIA / 3 mm THICK Al. DISC

38mm D.~ 38 mm DIA.}ss DISC ~ 0.8mm DIA. ,

/

TLD or SSNTD on Electret: Electret Charge Configuration and Electret Holder

18

7. References

111 Marvin H.B., Nucleonics 13, 82 (1955)

121 Wolfsen J.L. and Dyment J.C.; Health Physics 7, 36 (1961)

131 Sauser H. and Ronge W., Health Physics 34, 97 (1978)

141 Kotrappa P., Dua S.K., Gupta P.C. and Mayya Y.S., Health Physics 41, 35 (1981)

151 KotrappaP., Gupta P.C., DuaS.K. and Soman S.D., Radiation Protection Dosimetry, ln press (1982)

161 Sessler G.M. and West J.E., Rev. Scient. lnstrum. 42, 15 (1971)

171 Cowper G. and Davenport M.R., An instrument of long term average radon Ieveis, Proc. IAEA Sym-

posium on Advances in Radiation Protection Monitoring, IAEA-SM-229/31, p. 413 (1979)

181 Porstendörfer J. and Mercer T.T., Health Physics 37, 191 (1979)

191 Kotrappa P., Dua S.K., Pimpale N.S., Gupta P.C., Nambi K.S.V., Bhagwat A.M. and Soman S.D.,

Health Physics, in press (1982)

19

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